1. Field of the Invention.
This invention relates to the continuous production of .beta.-alumina ceramic articles by a sintering process in which compressed powder material is traversed through a furnace.
.beta.-Alumina ceramic is a material having a nominal composition by weight of 5% Na.sub.2 O, 95% Al.sub.2 O.sub.3. The amount of sodium oxide in practice can range from 5 up to 10%. It may also contain oxides of magnesium and/or lithium. The material can be sintered in the temperature range 1550.degree. to 1900.degree. C. It is used in sodium sulphur cells and other electrochemical devices requiring passage of sodium ions. Desirable properties for this material in such applications include high density, imperviousness to helium gas and close control of composition and properties throughout the bulk and particularly right up to the surface of the material. The material may typically be required in the form of long thin wall tubes with or without a closed end or in the form of discs.
2. Prior Art.
As is described in British Patent Specification No. 1297373, articles of .beta.-alumina ceramic may be produced using a fast firing technique by forming shapes of compressed powder of the required composition and moving these shapes through a tubular furnace so that a short length of the material is raised to the sintering temperature, the movement being continuous so that the heated zone is gradually moved along the length of the material to be fired. The compressed powder may be pre-dried powdered .beta.-alumina ceramic or a pre-dried powdered mixture of materials which, on heating, produce .beta.-alumina ceramic.
At the firing temperature, the volatile component (Na.sub.2 O) vaporises, leading to variability of composition and ceramic properties in the article. In conventional firing, loss of soda may be prevented by buffering, i.e. by surrounding the ceramic with a loose powder of the same composition. However due to temperature gradients within the furnace causing variations in the vapour pressure of the soda, the fired ceramic may still have properties which vary between different parts of the article.